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RWMA
Q&A
BY ERIC PAKALNINS
Q:
Our company is in the process of
quoting the resistance spot welding of
several small automotive assemblies.
We must provide the customer with
metallographic results using AWS
D8.1M:2007, Specification for Automotive
Weld Quality — Resistance Spot Welding of
Steel. This standard contains a large
amount of information. Could you review
and discuss the metallographic information I need to know to satisfy my customer’s request?
Fig. 1 — Fusion zone, nugget width, HAZ, penetration, and indentation (Ref. 1).
A: Before discussing the metallographic
requirements of AWS D8.1M:2007 (Ref.
1), it is important to understand the specification’s scope. This specification applies to resistance spot welds in automotive steels and is intended to define their
desired weld quality. It does this by addressing the needs of both the production floor and the laboratory. Welding
experts from various automotive and
steel companies, and consulting/testing
organizations agreed to the quality characteristics and metrics pertaining to these
resistance spot welds. The evaluation
methods and inspection criteria specified
in this standard include both nondestructive and destructive elements and can be
used to 1) evaluate welding equipment
capability and procedures (such as weld
schedules), and 2) characterize the weldability of a particular steel. Further, Section 5 (Spot Weld Acceptance Criteria)
states that not all inspection methods
have to be applied to determine weld
quality. In other words, it is up to the customer to determine what test methods
are appropriate to define weld quality for
a particular application.
Before discussing the metallographic
information your customer has requested, it is important to note that
D8.1M:2007 classifies steels used in resistance spot welding by their minimum
ultimate tensile strength (UTS). Table 1
in the specification has steels grouped
into four categories: Low Strength (<350
MPa), Intermediate Strength (350 to 500
MPa), High Strength (>500 MPa up to
and including 800 MPa), and Ultra High
Strength (>800 MPa). The specification
does not require knowledge of what
group the steels are from when metallographically inspecting the resistance spot
weld(s). However, it is recommended to
have this information. Please note that
it is necessary to know the steel classification when performing nondestructive
surface inspections and other destructive
inspections such as peel/chisel tests and
shear/cross-tension load tests.
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MARCH 2013
Fig. 2 — Example of discrepant pores and cavities (Ref. 1).
Fig. 3 — Example of discrepant internal cracks (Ref. 1).
Destructive Inspection and
Metallographic Criteria
(Section 5.2)
Sample Preparation
The welds to be evaluated should be
cross-sectioned slightly off-center so that
after mounting (if necessary) and sanding/polishing, the centerline plane going
through both sheets will be available for
examination. You should check with the
customer to determine the desired crosssection orientation as weld(s) can be
cross-sectioned in any direction, including parallel (longitudinal cross section)
or perpendicular (transverse cross section) to the edge of the sample. After obtaining the necessary surface finish, the
customer may want photomacrographs
(typically 10 to 20×) prior to etching.
These can be examined for the presence
of cracks and porosity. Next, the weld is
exposed to a suitable etchant to bring out
the weld features (weld nugget, heataffected zone) and base metal.
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Nugget Width (Section 5.2.1)
Weld nugget width is one of the main
parameters to be determined from metallographic examination. As shown in
Fig. 1, the nugget width is the width of
the fusion zone. The fusion zone is the
volume of solidified metal resulting from
the welding process. It does not include
the heat-affected zone (HAZ). The HAZ
is the portion of base metal next to the
nugget whose mechanical properties or
microstructure have been altered by heat
input from the welding process.
The term “nugget” is often incorrectly
used interchangeably with the term “button.” A button is the part of a spot weld
that tears out during destructive testing
of welded steel. It may include all or part
of a nugget, the HAZ, and base metal.
Typically, a hole is left in the mating
sheet(s).
An acceptable weld has a nugget
width equal to or greater than the weld
size listed in Table 2 of the specification,
unless the customer specifies otherwise.
This weld size is also called the minimum
weld size (MWS). The MWS is approximately related to 4 × √t, where t is the
governing metal thickness (GMT). Table
2 has a range of GMTs called out for each
listed weld size.
The GMT for a two-sheet (2T) stackup is the thinner metal thickness of the
two sheets. For a three-sheet (3T) stackup, the GMT is generally the metal thickness of the second-thickest sheet. However, another approach for a 3T stack-up
is to consider each of the two faying surfaces as a separate 2T thickness where
the GMT would be the thinner metal
thickness of the two sheets making up the
faying surface. Consult your customer to
determine their weld size requirement
for a 3T stack-up.
Table 2, note b, states that the weld
size applies to both the nugget width obtained metallographically or to the size
of the fracture mode (for example, a button) that is obtained from destructive
testing. It is possible that a customer may
specify one weld size requirement for a
nugget and a different weld size requirement for a fracture mode result obtained
from destructive testing of the same weld.
When measuring the nugget width in
Fig. 1, the customer may require that any
unfused lengths within the nugget where
the faying surface or interface between
the two sheets originally existed, are subtracted from the nugget width to come
up with an effective nugget width. Check
with your customer to determine their
requirement for this situation.
Penetration (Section 5.2.2)
Penetration is the ratio of the nugget’s
maximum depth of fusion to the pre-
welded sheet thickness, expressed as a
percentage [(P1/t1) × 100 and (P2/t2) ×
100]. See Fig. 1 for 2T welding. It should
exceed 20% of the prewelded sheet thickness into each sheet of the stack-up for
both 2T and 3T welding. One hundred
percent penetration into the outer sheets
is undesirable, and in that case, it
may result in discrepancies such as
surface cracking.
Indentation
D8.1M:2007 discusses indentation in
Section 5.1.4 (Surface Inspection Criteria). Indentation is the ratio of the
amount of weld depression to the
prewelded sheet thickness, expressed as
a percentage. Indentation should be less
than 30% of the thickness of each outside sheet of the welded joint unless specified otherwise by your customer.
Although indentation is not addressed
in Section 5.2 (Destructive InspectionMetallographic Criteria), it is possible to
measure indentation depth as shown in
Fig. 1. For 2T welding, the percentage indentation into each sheet t1 and t2 can
then be calculated. If the indentation
depth varies over the length of indentation width, it is usually measured at its
deepest location. Verify this with your customer. 3T weld surface indentations are
calculated the same way as the 2T weld
surface indentations by considering the
outer two sheet thicknesses to be t1 and
t2. A final note of caution: ensure that
sheet metal deformation is not included
in the measurement of indentation.
makes the weld in Fig. 2 discrepant.
Internal Cracks (Section 5.2.4)
Welds are discrepant if the cracks observed in the nugget or HAZ at 10× meet
any one of the criteria listed below:
Calculate the length of each crack in
the fusion zone only (see Fig. 3).
1. If the sum of all the lengths (L1 +
L2 + L3) is equal to or greater than 25%
of the nugget width, then the weld is
discrepant.
Calculate 15% of the linear distance
between the fusion zone periphery and its
center. Measure this distance inboard of
the fusion zone periphery to establish a
15% inboard of fusion zone periphery as
shown by the dotted line in Fig. 3.
2. If any crack or portion thereof (for
example, the crack defined by L1) extends into this outer zone between the
fusion zone periphery and the 15% inboard of fusion zone periphery, then the
weld is discrepant. Also, if there are any
cracks in the HAZ (for example, the
crack defined by L4), the weld is discrepant. An exception to this would be
cracks deemed acceptable by surface inspection (refer to Section 5.1.2), but do
not extend into the fusion zone.
In summary, AWS D8.1M:2007 has
detailed acceptance criteria relating to
weld size, penetration, indentation,
porosity, and internal cracks as they pertain to metallographic analysis. However,
check with your customer to determine
whether they have modifications to these
requirements.◆
Reference
Porosity (Section 5.2.3)
Welds are discrepant if the pores, cavities, or voids observed in a cross section
examined at 10× meet any one of the criteria listed below:
Calculate the area and length for each
pore, cavity, and/or void (see example in
Fig. 2).
1. If the sum of all the areas (P1 + P2
+ P3) is equal to or greater than 15%
of the nugget area, then the weld is
discrepant.
2. If the sum of all the lengths (L1 +
L2+ L3) is equal to or greater than 25%
of the nugget width, then the weld is
discrepant.
Calculate 15% of the linear distance
between the fusion zone periphery and its
center. Measure this distance inboard of
the fusion zone periphery to establish a
15% inboard of fusion zone periphery as
shown by the dotted line in Fig. 2.
3. If any pore, cavity, and/or void or
portion thereof extends into this outer
zone between the fusion zone periphery
and the 15% inboard of fusion zone periphery, then the weld is discrepant. For
example, the pore defined by area P1
1. AWS D8.1M:2007, Specification for
Automotive Weld Quality — Resistance
Spot Welding of Steel. Doral, Fla.: American Welding Society.
ERIC PAKALNINS is a welding consultant with RoMan Engineering Services. He
is a member of the AWS Detroit Section
and serves on the D8D Subcommittee on
Automotive Resistance Spot Welding. He
retired as a senior specialist with the
Chrysler Materials Engineering Welding
Group. This article would not have been
possible were it not for the assistance from
members of the RoMan team. Send
your comments/questions to Eric at
[email protected], or to Eric
Pakalnins, c/o Welding Journal, 8669
Doral Blvd., Suite 130, Doral, FL 33166.
WELDING JOURNAL
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